This invention relates to methods of improving surface characteristics of extruded thermoplastic tubing, particularly such tubing for inclusion in medical devices in which the tubing is intended for intravascular insertion into the human body.
Tubular products which are intended for medical applications, and more specifically, which are to be introduced into the body making contact with body tissue, fluids or vessels are preferably made as smooth as possible in order to minimize friction and drag during bodily insertion. Most plastic tubular products, suitable for catheters and the like, are formed by an extrusion process so that the outside surface of the tubing has relatively smooth properties. In many instances, a standard extrusion process for producing hollow tubular plastic products can be optimized to produce a lubricity level on the outside surface of the tubing which is adequate for medical purposes. However, although this surface lubricity or smoothness may be acceptable from the operational standpoint, the level of surface smoothness produced by mere extrusion may still produce some discomforturn to the patient because of surface friction and drag during insertion.
Some efforts have been taken to improve surface lubricity of extruded thermoplastic tubing by applying silicone fluids or resins thereon. This, however, requires an additional operational step and increases expense of the product. Furthermore, cleanliness and possible contamination have to be considered inasmuch as the tube is intended for bodily insertion. Accordingly, improved techniques are still being sought for improving the surface characteristics of extruded thermoplastic tubing intended for medical purposes.
In U.S. Pat. Nos. 3,205,290; 3,108,851; and 3,089,187, plastic tubing is extruded and then stretched. The purpose of the stretching is to molecularly orient the material in order to increase its strength in either the longitudinal or transverse directions. In these patents, the stretching operation is part of or immediately following the extrusion step so that the plastic material is generally at an elevated temperature. There is no indication in these patents that the stretching, which causes molecular re-orientation, also produces an increase in smoothness or surface lubricity of the tubular product.